There has been much controversy over the injury mechanism that occurs in rear-end auto collisions. Some of the insurance companies have either tried to deny that anyone gets injured at all or have attributed the injury to strained muscles. However, recent research by Barnsley et al indicates that the source of the pain in a majority (~60%) of whiplash patients is the zygapophyseal (facet) joints, rather than the muscles.(1,2) The most common levels of zygapophyseal joint pain found in this study were at the C2/3 and C5/6 levels. These researchers used sophisticated injections of anesthetics into the facet joints under fluoroscopic guidance to determine this, though, and such injury to the soft tissue structures of the zygapophyseal joints (such as the joint capsules) are not easily seen with x-ray, CAT scans or MRI. But if needed, facet blocks could be done by anesthesiologists to prove these injuries in a medico-legal environment.

We know that many people in low speed collisions are injured and that a large number of these patients continue to report symptomatology for years after their accidents.(3,4) These patients continue to have pain long after their cases are settled, thus dispelling the myth that we are dealing with "litigation neurosis."(5,6) Barnsley, et al. reviewed the studies that have followed up on whiplash patients for a period of time after the injury and have come up with a conservative estimate of between 14 to 42% of whiplash patients who develop chronic neck pain and 10% who will have constant, severe pain indefinitely. (7)

A new study by Kaneoka et al (8) now provides a demonstration of the mechanism of the whiplash injury and how the zygapophyseal joints and other spinal structures are injured. Kaneoka used cineradiography of human volunteers while undergoing a simulated rear end collision and found an unusual pattern of movement of the cervical vertebrae. The lower cervical segments rotate around an abnormally high instantaneous axis of rotation. In particular, C6 starts to extend before the rest of the spine. This results in an s-shaped configuration of the spine as C6 extends and the rest of the spine initially flexes. Then, as the rest of the cervical spine extends, this results in an extreme rotational torque at the C5/6 level. This results in a stretching of the anterior longitudinal ligament and forces the inferior facet of C5 to collide with the superior facet of C6. This may result in impingement and inflammation of the folds of synovial tissue between the zygapophyseal joints (facet synovial fold impingement syndrome). And Giles has previously demonstrated that these synovial folds are pain sensitive.(9)

In conclusion, we now have evidence that the anatomical structures most commonly injured in low speed whiplash injuries are the zygapophyseal (facet) joints and we have a mechanism to explain how this injury occurs. And such a joint injury cannot always be expected to heal within 6 weeks and could explain why some whiplash patients continue to experience pain for long periods of time after their accidents and why they have a higher incidence of degenerative spinal changes in the cervical spine years later. Also, since chiropractors can specifically treat the zygapophyseal joints with specific manipulative techniques, we may be the most qualified professionals to treat these injuries. In fact, several researchers have hypothesized that one of the most important benefits of manipulation is freeing up entrapped folds of synovial tissue between the zygapophyseal joints.(10,11)


1. Barnsley L, Lord SM, Wallis BJ, Bogduk N. The prevalence of chronic cervical zygapophyseal joint pain after whiplash. Spine. 1995; 20:20-5.
2. Lord SM, Barnsley L, Wallis BJ, Bogduk N. Chronic cervical zygapophyseal joint pain after whiplash. A placebo-controlled prevalence study. Spine. 1996; 21: 1737-1745.
3. Macnab I. Acceleration extension injuries of the cervical spine. In Rothman RH, Simeone FA (eds): The Spine, ed 2. Philadelphia, WB Saunders. 1982; Vol. 2: p.648.
4. Foreman SM, Croft AC. Whiplash Injuries. Baltimore: Williams and Wilkins, 1988, p.323.
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7. Barnsley L, Lord SM, Bogduk N. Whiplash injury. Pain. 1994; 58:283-307.
8. Kaneoka K, Ono K, Inami S, Hayashi K. Motion analysis of cervical vertebrae during whiplash loading. Spine. 1999. Vol. 24: 763-770.
9. Giles LGF, Harvey AR. Immunohistochemical demonstration of nociceptors in the capsule and synovial folds of human zygapophyseal joints. Br J of Rheumatol. 1987; 26: 362-364.
10. Bogduk N, Jull G. The theoretical pathology of acute locked back: a basis for manipulative therapy. Man Med. 1985; 1:78-82.
11. Giles LGF. Pathoanatomic studies and clinical significance of lumbosacral zygapophyseal (facet) joints. JMPT. 1992: 15:36-40.